1. Field of the Invention
The present invention generally relates to a wireless communication system, and in particular, to an apparatus and method for supporting an Envelope Tracking (ET) power amplifier in a transmitting end that uses power control in a wireless communication system.
2. Description of the Related Art
As a standard of a next-generation wireless communication system (e.g., Institute of Electrical and Electronics Engineers (IEEE) 802.16m, 3rd Generation Partnership Project 2 (3GPP2) Long Term Evolution (LTE), etc.) is defined based on an Orthogonal Frequency Division Multiplexing (OFDM), an efficiency deterioration problem caused by a high Peak to Average Power Ratio (PAPR) which is a characteristic of the OFDM is becoming a big issue. To overcome this problem, various schemes such as Doherty, Envelop Tracking (ET), Envelope Elimination and Restoration (EER), etc., are being researched and developed.
The Doherty scheme is a technique in which peaking amplifiers are connected in parallel and efficiency is increased by using load modulation. Although the Doherty scheme can be implemented relatively simply, an amount of improvable efficiency is limited. In comparison thereto, the EER scheme is a technique in which a bias of a power amplifier is dynamically controlled along an envelope of an input signal, and can achieve the highest efficiency improvement. However, to implement the EER amplifier, in the case of an LTE system that uses a signal having a frequency 10 to 20 times higher than that of an envelope input signal, i.e., a signal having a frequency of 20 MHz, an Envelope Modulator (EM) for modulating a drain (or collector) bias of the power amplifier with significantly high efficiency is required when a signal having a frequency higher than 200 to 400 MHz is used. However, there is a difficulty in that its implementation is almost impossible in practice or implementation costs are significantly high.
The ET scheme is a technique which can be implemented with sufficiently high efficiency in practice as a compromise between the Doherty scheme and the EER scheme. The ET scheme uses a power amplifier having linearity without having to completely eliminate the envelope as in the EER scheme, and thus a signal having about a 3-fold frequency, not a 10-fold frequency, can be used as an input signal of the EM.
The ET scheme has generally been used by a Base Station (BS) due to its relatively complex structure. However, as Complementary Metal-Oxide Semiconductor (CMOS) technologies have been recently developed and a cheap and easy digital circuit implementation has become possible, an attempt for applying the ET scheme to a user terminal, a Consumer Premise (CP), etc., has been taken into account.
A biggest difference between the BS and the terminal lies in whether power control is applied or not. In the case of the BS, transmit power is constant, whereas in the case of the terminal, power control is persistently performed according to a channel environment. In the currently used ET scheme, an operation of the power control environment is limited, and if relatively small power is used in comparison with the BS, such as in the case of using the terminal or the like, power consumption of the EM and power consumption of an extra device added for an operation of the ET scheme may become greater than a power amount saved in the power amplifier. Accordingly, in order to apply the ET scheme to a transmitting end (e.g., the terminal or the like) that performs power control by using low power, there is a need to overcome the aforementioned problems.